The present study is investigating a possible role for morphine and, by extension, endogenous opiods, in the motivation and control of playful behavior in squirrel monkeys. Play is a relatively fragile behavior which is easily disrupted by many stressors and by a variety of drugs, none selectively. Enhancement of play, selective or otherwise, has been a similarly difficult objective to achieve. Morphine is the sole pharmacologic agent with demonstrated ability to increase rates of play, based on work with juvenile rats, where both frequency and content of play are affected. Squirrel monkeys were chosen for the present studies because our laboratory has established an extensive and exhaustive database on this very playful primate species, giving us a great advantage in detecting deviations from the norm at many levels. Using as subjects pairs of squirrel monkeys who have demonstrated robust play, we found that low doses of peripherally administered morphine (0.5 mg/kg) increased the incidence of play initiation behaviors by the treated animal, indicating increased motivation to play. The duration of play bouts increased by about 30% over baseline levels when one member of the pair received a morphine dose.